1. Field of the Invention
The present invention relates to an image forming device wherein printing is performed by scanning the device across a recording medium.
2. Description of the Related Art
British Patent Specification 1 366 253 describes a conventional manual image forming device with a displacement detection unit for detecting relative positional change between a print head of the device and a recording medium in order to determine the scanning speed of the manual image forming device across the recording medium. In order to detect relative positional change generated by the print head moving across the recording medium, the detection unit has a roller which moves in correspondence with movement of the print head by rotating in contact with the surface of the recording medium. The detection unit detects rotational angle of the roller to determine relative positional change between the print head and the recording medium and, consequently, the position of the print head.
In this way, even when the scanning speed is not uniform, the conventional image forming device can perform excellent printing by using the displacement detection unit to detect relative positional change between the print head and the recording medium and, at the same time, controlling the print head based on the detected results. Further, the image forming device having the above-described configuration can easily print on any position of the recording medium and can print on a variety of recording media, such as on thick prebound books or documents, or large-sized sheets.
However, because this type of image forming device is configured so as to be manually scanned, the user may scan the device too quickly so that the maximum drive frequency of the print head is exceeded. In this case, the print head can not be driven rapidly enough to keep up with the scanning speed of the device so that the resultant printed image has poor quality. In order to solve this problem, U.S. Pat. No. 4,851,921 discloses a device wherein an alarm rings to warn a user when the present scanning speed exceeds a preset speed. Printing can be properly performed using the device of U.S. Pat. No. 4,851,921, as long as the user scans the device at a speed within a range where the alarm does not ring.